1. Field of the Invention
This invention relates to an LED array printhead which is used in an electrophotographic printer for example. The present invention also relates to a method of making such a printhead.
2. Description of the Prior Art
A typical LED array printhead comprises a head substrate which carries an array of semiconductor LED chips and an array of drive IC's connected to the respective LED chips via wires, as disclosed in Japanese Patent Application Laid-open No. 2-184466 for example. The respective LED chips have a multiplicity of light emitting portions to provide a line of light emitting dots, and the respective dots are selectively actuated by the drive IC's for light emission.
In such an printhead, it is necessary to protect the LED chips together with their associated components which are sensitive, thereby preventing these components against deterioration. One way of such protection is to enclose the entire printhead in a protective case. However, use of the protective case results in an increase of size and cost in addition to complicating the structure.
Japanese Patent Application Laid-open No. 3-34475 proposes an LED array printhead wherein use is made of a transparent protective resin coating for covering at least an array of LED chips. Obviously, such an arrangement eliminates the problems caused by the use of a bulky protective case.
However, in any LED array printhead, it is necessary to check the luminance of the respective light emitting portions in order to ensure an acceptable printing quality. If one or more of the light emitting portions cannot meet a minimum luminance level, the relevant LED chip must be removed from the head substrate for replacement by a new one, thus resulting in a cost increase. Further, in case a protective resin coating is formed to cover the LED chip array, the resin coating must be partially removed and then formed again after replacement of an unacceptable chip, consequently requiring a longer time than when no resin coating is provided. Moreover, when the thickness of the resin coating is uneven longitudinally of the printhead, the thickness unevenness of the resin coating itself adds to the unevenness in luminance distribution of the printhead, thus additionally deteriorating the printing quality.